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Sticky tape is not only see-through, it can also help you see through objects by emitting x-rays.

Peeling tape from a roll of sticky tape releases tiny bursts of x-rays that are powerful enough to take images of bones in fingers and hands, according to a study appearing in the latest issue of Nature.

The unusual discovery was made by a University of California at Los Angeles team, intrigued after hearing that Soviet scientists in the 1950s found that sticky tape, when separated at the right speed, released pulses in the x-ray part of the energy spectrum.

The investigators used a motorised peeling machine to unwind a standard roll of Photo Safe 3M Scotch tape at a speed of three centimetres per second.

By placing the machine in a vacuum, they were able to measure x-rays that were enough to take images.

"We took some pictures of our hands to see the bones and prove that it was possible. We have a whole collection (of pictures)... it is absolutely remarkable."

As to precisely why this phenomenon occurs, Camara admits: "That is a mystery."

Triboluminescence

The core theory is something called "triboluminescence," which occurs when two contacting surfaces move relative to each other.

As the tape peels, the sticky acrylic adhesive, on the back of the tape, becomes positively charged, while the polyethylene roll becomes negatively charged, according to this hypothesis.

At a reduced atmospheric pressure, the mechanical act of pulling apart the two surfaces causes electric fields to build up that then trigger discharges of energy.

This accelerates electrons on the adhesive to very high speeds and when they whack into the positively-charged tape roll, x-rays result. The pulses last for a billionth of a second, with an intensity of 100 milliwatts.

"It is a discharge process," says Camara. "It is not clear exactly what mechanism is taking place, but electrons are plainly flying from one side to the other at very high speed and when they hit the other side and they stop, x-rays are emitted.

He added that there could one day be a useful outlet to the discovery - x-ray machines that would be cheaper and safer than conventional machines that require a permanent radioactive source and a highly trained technician for maintenance.

"There's lots of room for improvement" from the basic discovery, says Camara.

"Just peeling tape is the quickest, cheapest way to provide x-rays... It's x-rays for everyone."